The basics of vacuum pre-condenser barometric legs

In the complex world of downstream oil and gas processing, the barometric leg is a simple yet essential component of a vacuum system that employs a pre-condenser. The leg itself has these essential principles of design:

34-foot length in a straight vertical run is always recommended.

If 34 feet of straight vertical run is not available, a 45-degree-angle turn is acceptable.

Barometric leg extends 6-8 inches into the hotwell to create a seal.

A barometric leg is basically a condensate drain that consists of a leg pipe between the pre-condenser and the hotwell.

Valuable hydrocarbons or water are common constituents in oil and gas downstream processes. In vacuum pump system design, pre-condensers should be considered if a large part of the gas load is condensable. Precondensers can recover these constituents and protect the vacuum pump from potential process upsets. Also, pre-condensers may allow the use of a smaller vacuum pump, reducing total capital and energy usage.

A barometric leg is basically a condensate drain that consists of a leg pipe between the pre-condenser and the hotwell. The condensate drains by gravity and drops down through the leg into the hotwell, where the condensate is eventually collected.

Direct-contact condensers (pre-condensers) are often referred to as barometric condensers because they normally drain the condensate via a barometric leg. Direct contact of the process gas with the cooling fluid removes heat and allows condensation of the process gas. Surface condensers do not allow direct contact between the condensable gas load and the coolant. Rather, separate compartments are used for the coolant and the condensable gas load, and heat is transferred through the dividing wall. Whatever vacuum pre-condenser type is being used, the barometric leg is routinely employed.

Since the condensate is under vacuum in the condenser and it is trying to move toward a hotwell that is under atmospheric pressure, the pressure difference is going the wrong way. The condenser must be located higher than the hotwell to allow enough static head pressure of the condensate to overcome the pressure differential.

Every inch of mercury absolute (HgA) will raise a column of water 1.13 feet, so a leg of 34 feet will allow the operating vacuum pump to near perfect vacuum while allowing water and condensate to drain as required. The leg assembly should be designed for a straight vertical run or one with no more than a 45-degree turn; 90-degree turns are not acceptable.

The system is effectively sealed by submerging the leg pipe 6-8 inches below the hotwell liquid level, so atmospheric air/vent gases can't be "pulled" back into the piping. If you vent into a system under any pressure, the pressure differential will increase, requiring a taller barometric leg.

Improper barometric leg design will reduce the performance of the condenser. Should the condensate overcome the effects of gravity due to poor design, the condensate will flow back into the condenser and flood the lower tubes. If the tubes flood, they will not be able to transfer heat effectively.

The length and vertical requirements can be a challenge to overcome when so many other processing components are in the way. In one instance of a design flaw, the leg started vertically from the top, but after about 10 feet of tailpipe, there was an obstacle. The designers attempted to bypass it by running the leg at a 90-degree angle and then returning to vertical orientation. As a result, the leg would not drain. The condensate in the pipe stayed in the pipe. With a 90-degree angle in the tailpipe, it would be necessary to start over from that point and regain the necessary 34-foot length to the hotwell.

For more information on barometric legs and vacuum solutions for downstream oil and gas processing, visit www.dekkervacuum.com or call (888) 925-5444.